Kombo: One aspect of gaming that I suspect will always challenge designers -- and sometimes gamers, ironically enough -- is properly tuning the game difficulty.
Not all games should be easy, not all games should be hard. And not all games should be held to the same standard; different types of games demand different design.
Two games from late last year on opposite ends of the spectrum but well tuned all the same: Mega Man 9 and Prince of Persia. Both were spoken of frequently for their difficulty.